Generally, balance is a huge factor in choosing a board and the taller you are or the heavier you are plays a huge role in selection of your board. Ideally, the heavier you are the wider the board should be. Balance and strength of the board play a key roll in the selection. Larger and taller individuals may have bigger feet and therefore need a wider board, say 9 inches or wider.
All of this just means you will fall harder so you should be wearing pads. Elbow, knee, shin, and wrist pats would help prevent injuries. Even if you want to skate with a smaller board to do tricks and for pool or ramp riding you'll need more practice than the average guy. I'd even suggest wearing padded shorts, they will save those bones from bruises. If you think pads make you look lame, think what you may look or feel like with the skins, and broken bones you may get by not wearing them, a clear choice! And this along with not forgetting to wear that all important brain protection, the helmet!
Okay, after all of this, the size of the board you select is in accordance with the task you plan to use it for. You can still use a smaller board for ramp, pool, or park riding, but you may have to be more skillful and work harder at perfecting your ride. A longer wider board will provide better balance for someone over 6 '. And remember to get the right wheels and trucks for higher speeds such as downhill and street riding,
All in all, skating is fun, you'll have a great time, and you will fall more, but if you are in the right gear, you can save yourself a lot of pain and maybe even you life.
In closing, know your board, choose wisely.
Choose the right board for the purpose you want to use it for.
Do your research on which brands are best suited for the purpose you intend to use them for.
Know the strength of the deck and how well its made. If you are in doubt talk to someone from you local skateboard shop, the website of the manufacturer of the skateboard, or go to a skateboarding forum on the internet to get information.
All of the above will insure that you have done all you can to have the experience of enjoying the sport of skateboarding.